Provide a way to disable use of extern templates in libc++.  This is intended for the clients of libc++, not the libc++ build.  The dylib should always contain the extern templates.  To disable the client needs to put -D'_LIBCPP_EXTERN_TEMPLATE(...)=' on the command line.

git-svn-id: https://llvm.org/svn/llvm-project/libcxx/trunk@167486 91177308-0d34-0410-b5e6-96231b3b80d8
diff --git a/include/string b/include/string
index 00016ee..5bf42f0 100644
--- a/include/string
+++ b/include/string
@@ -1031,7 +1031,7 @@
 #pragma warning( push )
 #pragma warning( disable: 4231 )
 #endif // _MSC_VER
-extern template class __basic_string_common<true>;
+_LIBCPP_EXTERN_TEMPLATE(class __basic_string_common<true>)
 #ifdef _MSC_VER
 #pragma warning( pop )
 #endif // _MSC_VER
@@ -3975,8 +3975,8 @@
 
 #endif  // _LIBCPP_HAS_NO_RVALUE_REFERENCES
 
-extern template class basic_string<char>;
-extern template class basic_string<wchar_t>;
+_LIBCPP_EXTERN_TEMPLATE(class basic_string<char>)
+_LIBCPP_EXTERN_TEMPLATE(class basic_string<wchar_t>)
 
 extern template
     string